The detection of the variations in the reactivity of the core of a pressurized water nuclear reactor forms one of the means for controlling the correct operation of the reactor.
It is known to detect the variations in the reactivity of the fuel of a reactor from the reactor power, this power, which can be measured, for example, with the aid of flux chambers which are external to the core, being a function of the reactivity of the fuel.
However, when using such processes, it is not possible to distinguish the reactivity variations induced by the changes in the mean temperature of the primary fluid in the core from the reactivity variations due to a defective location of the control clusters, for example an untimely fall of clusters. Now, it is highly important to know the reactivity variations attributable to the clusters, since these variations are frequently very considerable and require prompt intervention by the supervising staff. Moreover, if it is desired to be able to act as promptly as possible, the reactivity variations must be detected very rapidly, even in a transient period.